1. Field of the Invention
This invention relates to radiant energy sensor systems and more particularly to an infrared sensor system employing a differential amplifier for removal of background radiation from the detected infrared radiation.
2. Description of the Prior Art
When thermal or infrared images are detected such as, for example, when the earth is viewed from a satellite in space, usually a relatively narrow continuous strip of the earth's surface is detected as the satellite orbits the earth. The infrared radiation from a target scene, detected by a sensor, contains background radiation energy which is on the order of 98% of the total target scene radiant energy detected. Infrared sensors made from extrinsic silicon such as Si:In or Si:Ga can take advantage of the well-developed silicon device design and processing techniques. Charge coupled device circuits are quite useful for signal processing in imaging applications and have been coupled with extrinsic silicon detectors to make an infrared imaging device. However, the charge-carrying capabilities of charge coupled devices (CCD) are limited. Target scene radiation energy containing the infrared background radiation levels can saturate the capacity of a CCD read-out register.
Previous attempts have been disclosed for removal of background radiant energy from the target scene. In U.S. Pat. No. 4,064,533, by D. R. Lampe et al. which issued on Dec. 20, 1977 and was assigned to the assignee herein, a CCD focal plane processor is disclosed having a plurality of columns of individual sensor elements with plural sensor elements per column. The structure includes plural CCD shift registers corresponding to the number of columns of sensors wherein each CCD shift register included a pair of stages corresponding to each of the element sensors. Two "snapshots" of a scene are taken at time-displaced intervals and are compared to detect differences therebetween to eliminate background or unchanging scene content and retain information regarding changes in scene items between shapshots. The CCD arrays must, however, handle the full amount of the signal charge generated.
U.S. Pat. No. 3,806,729, by J. M. Caywood which issued on Apr. 23, 1974, and U.S. Pat. No. 3,808,435, by R. T. Bate et al. which issued on Apr. 30, 1974, respectively, disclose an infrared imaging system which include an array of rows of CCD bits which collect charge alternatively from the target scene and from a uniform background reference source. A chopper is employed to provide the input to alternatively expose the CCD bits to the scene radiation and to the reference source and, by comparison, remove background radiation. On both patents, the CCD array must handle the full amount of the signal charge generated. U.S. Pat. No. 3,883,437, by K. Nummedal et al. which issued on May 13, 1975, discloses an image-scanned array of infrared radiation detectors wherein the signals are injected directly into a set of CCD registers for subsequent processing. Here, again, the CCD array must handle the full charge generated. U.S. Pat. No. 4,115,692 by R. S. Balcerak et al. which issued on Sept. 19, 1978, discloses a solid state pyroelectric detector read out device comprised of a two-dimensional pyroelectric detector array which is interfaced with an array of CCD's. The pyroelectric detector is utilized to reject background radiant energy from the detected radiation.
In U.S. Pat. No. 3,653,014 by W. F. List which issued on Mar. 28, 1972 and assigned to the assignee herein, apparatus is disclosed for enhancing small signal information by processing signals from a sensor array. The output levels of adjacent sensors are compared at the conclusion of each sampling interval by application to a differencing circuit followed by a logarithmic amplifier.
It is desirable, therefore, to provide a differential amplifier to remove radiant energy representing background and unchanged scene content from the detected radiation of a pair of infrared sensors and to retain only the differences between the radiation energies before transferring the signals from the sensor pair to a CCD read out register. Thus, the information related to constant signals corresponding to background radiation and unchanging scenes are removed.